Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm and include pacemakers and implantable cardioverter/defibrillators (ICD's). A pacemaker is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate. Pacing therapy may also be applied in order to treat cardiac rhythms that are too fast, termed anti-tachycardia pacing. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality, regardless of any other functions it may perform such as the delivery cardioversion or defibrillation shocks to terminate atrial or ventricular fibrillation.)
Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. For example, the heart pumps more effectively when the chambers contract in a coordinated manner. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses from the sino-atrial node to the atrial myocardium, to the atrio-ventricular node, and thence to the ventricular myocardium to result in a coordinated contraction of both atria and both ventricles. This both synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart's pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways, such as bundle branch blocks, can thus suffer compromised cardiac output.
Heart failure refers to a clinical syndrome in which an abnormality of cardiac function causes a below normal cardiac output that can fall below a level adequate to meet the metabolic demand of peripheral tissues. It usually presents as congestive heart failure (CHF) due to the accompanying venous and pulmonary congestion. Heart failure can be due to a variety of etiologies with ischemic heart disease being the most common. Some heart failure patients suffer from some degree of AV block or are chronotropically deficient such that their cardiac output can be improved with conventional bradycardia pacing. Such pacing, however, may result in some degree of uncoordination in atrial and/or ventricular contractions because pacing excitation from a single pacing site is spread throughout the myocardium only via the much slower conducting muscle fibers of either the atria or the ventricles, and not the specialized conduction pathways. Most pacemaker patients can still maintain more than adequate cardiac output with artificial pacing, but the diminishment in cardiac output may be significant in a heart failure patient whose cardiac output is already compromised. Intraventricular and/or interventricular conduction defects are also commonly found in heart failure patients and can contribute to cardiac dysfunction by causing unsynchronized contractions during intrinsic beats. In order to treat these problems, cardiac rhythm management devices have been developed which provide electrical pacing stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy.
One form of cardiac resynchronization therapy is biventricular pacing in which paces are delivered to each ventricle during a paced cardiac cycle separated by a specified resynchronization offset interval. The paces may be delivered in accordance with a bradycardia pacing mode in order to both resynchronize the ventricles and maintain an adequate heart rate. Patients treated in this manner may also occasionally suffer from ventricular tachyarrhythmias such as ventricular tachycardia. The implanted pacemaker may therefore also be programmed to deliver anti-tachycardia pacing therapy when a ventricular tachycardia is detected. Conventional anti-tachycardia pacing, however, involves pacing at only a single site of one ventricle, and the beneficial effects of resynchronization therapy upon cardiac output are lost while the anti-tachycardia pacing therapy is being delivered. The resulting diminishment in cardiac output may also lessen blood flow to the myocardium and increase the chances that the ventricular tachycardia degenerates into fibrillation.